Sodium carbonate (Na2CO3), or soda ash, is one of the largest volume alkali commodities made world wide with a total production in 2008 of 48 million tons. Sodium carbonate finds major use in the glass, chemicals, detergents industries, and also in the sodium bicarbonate production industry. The main processes for sodium carbonate production are the Solvay ammonia synthetic process, the ammonium chloride process, and the trona-based processes.
Trona ore is a mineral that contains up to 99% sodium sesquicarbonate (Na2CO3.NaHCO3.2H2O). Trona-based soda ash is obtained from trona ore deposits in Green River (Wyo.), Turkey, China, and Kenya either by conventional underground mining techniques, by solution mining or lake waters processing. The trona-based sodium carbonate from Wyoming comprised about 90% of the total U.S. soda ash production.
A typical analysis of the trona ore in Green River is as follows:
TABLE 1ConstituentWeight PercentNa2CO343.4NaHCO334.4H2O (crystalline and free moisture)15.4NaCl0.01Na2SO40.01Fe2O30.14Insolubles6.3Organics0.3
Trona deposits contain highly diverse soluble impurities such as alkaline metal halides (sodium chloride, potassium chloride, sodium fluoride, . . . ), alkaline metal sulfates (sodium sulfate, potassium sulfate, . . . ), alkaline metal nitrate (sodium nitrate, potassium nitrate, . . . ), alkaline metal borate, alkaline metal phosphates, etc. . . . . Those highly soluble impurities are in various proportions depending on the geographic location of the deposits. In particular, sodium chloride and sodium sulfate may represent several percents or several tens percents of trona ore depending on the geographic location.
Trona deposits also include slightly soluble mineral or organic impurities. Examples of slightly soluble mineral are: alkali metal and alkali earth metal silicates, aluminates, titanates, vanadates, metallic compounds and salts. The organic impurities come from organic sediments that were captured during the formation of the deposits and that frequently have formed oil shales during geological aging. Both mineral and organic soluble impurities may also be partially generated during the trona processing in the mine or on surface operations. In particular thermal treatments, such as calcination, generally amplify the quantity of some soluble impurities such as sodium silicates, and sodium salts of organic compounds by thermal saponification.
Other “insoluble” or very slightly water-soluble mineral impurities found in trona or adjacent to trona deposits are generally mixtures of different minerals, the most frequent of which are calcite, dolomite, pirssonite, zeolite, feldspar, clay minerals, iron/aluminium silicates, and calcium sulfate.
Two main techniques well known in the art are used to recover trona ore from trona ore deposits. The first technique is a mechanical mining, also called conventional mining, such as a room and pillar panel operation or a longwall operation. The second technique is a solution mining recovering wherein trona is dissolved with water and recovered as a solution.
Among the several ways in which sodium carbonate can be recovered from trona ore that contains other salts and impurities, the most widely practiced is the so called “monohydrate process”. In that process a mined trona ore is crushed, then calcined into crude sodium carbonate, then leached with water, the resulting water solution is purified and fed to a crystallizer where pure sodium carbonate monohydrate crystals are crystallized. The monohydrate crystals are separated from the mother liquor and then dried into anhydrous sodium carbonate. Most of the mother liquor is recycled into the crystallizer. However, the soluble impurities contained in the trona ore, tend to accumulate into the crystallizer. To avoid build up of impurities, the mother liquor must be purged. The purge liquor, which represents important quantities for industrial monohydrate plants, is commonly sent to an evaporative pond, also called tailings pond. The significant quantity of alkali which is contained in the purge liquor is consequently lost. Moreover, the stocking of large quantities of purge liquors in evaporative ponds raise environmental problems, because of the scarce availability of new areas for stocking.
Variants to produce sodium carbonate from trona ore, in particular when solution mining is used, is:                either crystallizing refined sodium sesquicarbonate (sesqui) after evaporation of water, then calcining the sodium sesqui into soda ash;        or alternatively decomposing thermally (with steam) or calcine chemically (with caustic soda) the dissolved sodium bicarbonate to transform it into dissolved sodium carbonate, then evaporating the water in order to crystallize pure sodium carbonate monohydrate.        
In those variants, the soluble impurities contained in the trona ore, tend to accumulate also into the sesqui or the monohydrate crystallizers. To avoid the build up of impurities, the mother liquors must also be purged, raising the same environmental problems in evaporative ponds as the monohydrate process.
Sodium bicarbonate (NaHCO3), aside sodium carbonate, is another important alkali product with a wide range of applications including human food, animal feed, flue gas treatment, and chemical industries. The production of sodium bicarbonate is currently almost entirely made by the carbonation of solid or aqueous solutions of sodium carbonate with gaseous CO2 either produced in situ in the soda ash plants or purchased independently.
Several technical alternatives have been proposed to reduce the purge volume from soda ash plants.
US2003/0143149 discloses a process for recovering sodium-based chemicals from sodium carbonate streams such as recycle, purge, and waste streams from sodium carbonate crystallizers, mine water, evaporative pond water and sodium carbonate decahydrate deposits. The sodium bicarbonate from those streams is partially destroyed by a decarbonization and the resulting stream is fed mainly back to a sodium carbonate monohydrate crystallizer, and the remainder of the resulting decarbonized stream is fed to a sodium carbonate decahydrate crystallizer, from which purified decahydrate is recovered and recycled to monohydrate crystallizer and a purge concentrated in impurity such as sodium sulfate is disposed off. Although the purge reduction factor of this process is limited, because, when high concentration of impurities is reached, sodium carbonate and sodium sulfate form decahydrated mixed salts. And if high amounts of sodium sulfate are recycled back to carbonate monohydrate crystallizer, they generate burkeite crystals (Na2CO3.2Na2SO4) that are detrimental to sodium carbonate monohydrate quality.
US2004/0057892 discloses a process for the production of sodium carbonate and bicarbonate, according to a purge liquor from a monohydrate sodium carbonate crystallizer is introduced into a sodium carbonate decahydrate crystallizer and the purified decahydrate crystals are converted into sodium bicarbonate. It has been observed that this process is not efficient when the purge liquor, depending on the trona source, contains high levels of impurities. High levels of sodium chloride in the purge liquor prevent smooth crystallization of sodium carbonate decahydrate.
U.S. Pat. No. 2,926,995 discloses a process for producing sodium bicarbonate crystals from sodium carbonate solutions containing sodium chloride from electrolytic caustic soda-chlorine cells.
U.S. Pat. No. 7,507,388 discloses a process for the production of sodium carbonate and bicarbonate, from a pre-purified solution comprising bicarbonate which is first partially decarbonized and then used in both a sodium bicarbonate line and a sodium carbonate monohydrate line. The purge stream of the sodium carbonate monohydrate crystallizer is either sent into a mixed sodium carbonate decahydrate and sodium sesquicarbonate line wherein resulting filtrate is discarded as the final purge of the process or sent after dilution into a light soda ash line comprising an intermediate sodium bicarbonate carbonation step, the bicarbonate is separated from the filtrate, and this filtrate is also disposed as a final purge. The taught total amounts of generated purges is very high (1.28 t of purges per ton of dense soda ash) and corresponds to 6 to 15 weight percents of purged sodium carbonate per ton of produced dense soda ash.
US2009/0291038 (Solvay) discloses a process for the joint production of sodium carbonate and sodium bicarbonate crystals, according to which a solid powder derived from sodium sesquicarbonate such as calcined trona is dissolved in water, the resulting water solution is introduced into a crystallizer, wherein sodium carbonate crystals and a mother liquor are produced, part of the mother liquor is taken out of the crystallizer (purge of the sodium carbonate crystallizer) and is carbonized (carbonated) to produce valuable sodium bicarbonate crystals and a second mother liquor, the second mother liquor is optionally decarbonized (debicarbonated) and then sent to a storage pond. In this document, it is taught that the mother liquor used for sodium bicarbonate crystallization should contain preferably at least 175 g/kg of sodium carbonate and not more than 60 g/kg of sodium chloride, and not more than 20 g/kg of sodium sulfate. Consequently, the purge level of sodium alkali (carbonate or bicarbonate) sent to a pond is reduced compared to a decahydrate treatment of the purge but is still important and represents important volumes sent into ponds.
US2011/112298 discloses a method for extending the life of tailings ponds produced from purge streams containing sodium carbonate wherein the purge stream is treated with gaseous carbon dioxide, similar to the US2009/0291038 process, to produce sodium bicarbonate or sodium sesquicarbonate before being introduced in the pond. The produced sodium bicarbonate may be recovered before the introduction of the treated purge stream into tailings pond or recovered after its deposition into the pond. The document is silent on further valorizing the obtained aqueous purge when sodium bicarbonate is recovered.
However, there is still a need in the sodium carbonate and bicarbonate industry, taking into account sustainable development, to be able to further reduce the purge volume and reducing the loss of alkali in a simple way, without impairing operation conditions of the linked processes.